Many devices comprise multiple interconnected parts that have different coefficients of thermal expansion. When such parts are subject to a change in temperature stresses occur as a result of the different parts expanding or contracting at different rates. This can cause some parts to be excessively strained to the point of cracking, breaking or otherwise being deformed in a manner that adversely impacts the part's ability to function as intended.
As an example, FIG. 1A illustrates a flip chip 1, also known as controlled collapse chip connection or its acronym, C4, whereby an integrated circuit die 2 is electrically connected to the interconnecting substrate 3 by solder bumps 4. The solder bumps 4 electrically connect the die 2 to the substrate 3 through metallic pads 5 and 6. The solder bumps 4 also assist in mechanically bonding the die 2 to the substrate 3. The severe thermal expansion mismatch between the silicon die 2 and the substrate 3 will introduce significant thermal stresses in the package, especially in the small solder bumps 4. An underfill 7 is typically provided to fill the space between the die 2 and the substrate 3 to assist in mechanically bonding the parts together and to reduce the thermal stresses in the solder bumps 4. The underfill is typically a polymer material (e.g. filled epoxy).
During the development of electrical and mechanical devices it is customary to conduct testing on the devices to ensure their electrical and/or mechanical integrity is maintained over a host of expected operating conditions. For example, during the development of a flip chip the device may be subjected to cyclic changes in temperature over a range in which the device is expected to operate to determine if the solder bumps 4 sufficiently maintain their structure to reliably provide an electrical interconnect between the die 2 and the substrate 3. Such testing is vital for devices over a broad range of technologies.